I am so sorry for your loss, all of you. A year is such a short time and you've had all those difficult first anniversaries/ birthdays / Christmas etc to deal with. Your MIL, for whatever reason, sounds unable to support you in a meaningful way so I hope you are getting support from other friends and family. It sounds as if you've tried to help her, and that's lovely of you. It is not your job to always be her support however if it costs you and your children too much emotionally. If she has a spouse, other children, friends to turn to, that's great. She may need more formal professional help, I don't know.
You've had some good suggestions about moving the photos and breaking the crying kissing ritual you find so challenging to deal with. If you want to have a more positive relationship between the two of you and your children, is meeting on neutral territory an option, rather than her coming to your house with all its associated memories for her ? A planned activity like the children's' school play, sports day, cinema, swimming etc might be easier for you to tolerate with her.
A year is a short time and I do feel for her, but she risks damaging your ongoing relationship and alienating her grandchildren. Little ones probably want to do fun things with granny, not watch her cry, and older ones might start eye rolling and disengaging if she continues to behave like this in front of them in the long term. I'm not trying to be critical of her, she's coping in the only way she can maybe, but it will have an impact on how others view her and whether they want to spend time with her. People can be tolerant for so long before they burn out, I really don't mean that to sound harsh.
I wish you and your family all the very best for the future.